import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;



public class LetterCombinationsOfAPhoneNumber {
    /**
     *给定一个仅包含数字2-9的字符串，返回所有它能表示的字母组合。答案可以按 任意顺序 返回。
     * 给出数字到字母的映射如下（与电话按键相同）。注意 1 不对应任何字母。
     * 链接：https://leetcode-cn.com/problems/letter-combinations-of-a-phone-number
     * @param digits
     * @return
     */
    public List<String> letterCombinations(String digits) {
        List<String> ans = new ArrayList<String>();
        if (digits.length()==0)
            return ans;
        Map<Character,String> phoneMap = new HashMap<Character,String>(){
            {
                put('2',"abc");
                put('3',"def");
                put('4',"ghi");
                put('5', "jkl");
                put('6', "mno");
                put('7', "pqrs");
                put('8', "tuv");
                put('9', "wxyz");
            }

        };
        backtrack(ans,phoneMap,digits,0,new StringBuffer());

        return ans;

    }

public void backtrack(List<String> ans,Map<Character,String> phoneMap,String digits,int index,StringBuffer combination){
        if (index==digits.length()){
            ans.add(combination.toString());
        }else {
            char digit = digits.charAt(index);
            String letters = phoneMap.get(digit);
            int letterCount = letters.length();
            for (int i=0;i<letterCount;i++){
                combination.append(letters.charAt(i));
                backtrack(ans,phoneMap,digits,index+1,combination);
                combination.deleteCharAt(index);
            }
        }

}



}
